1. Field of the Invention
The present invention relates to a method for producing a Group III nitride-based compound semiconductor device. More particularly, the present invention relates to a method for producing a Group III nitride-based compound semiconductor device employing the so-called laser lift-off technique, in which a Group III nitride-based compound semiconductor is epitaxially grown on a substrate made of a material different from Group III nitride-based compound semiconductor (hereinafter the substrate may be referred to as a “hetero-substrate”) to form a device structure; a conductive support substrate is bonded, via a conductive layer (e.g., a metal layer or a solder layer), to the uppermost layer of the device structure; and the hetero-substrate is removed by decomposing, through laser irradiation, a Group III nitride-based compound semiconductor thin layer in the vicinity of the interface between the Group III nitride-based compound semiconductor and the hetero-substrate. The present invention is particularly effective for a method for producing a Group III nitride-based compound semiconductor light-emitting device having an n-type layer and a p-type layer which sandwich a pn junction structure or an active layer.
2. Background Art
By virtue of a laser lift-off technique initially demonstrated by Kelly et. al. and described in Appl. Phys. Lett., vol. 69, 1996, p.p. 1749-1751, a light-emitting device or another Group III nitride-based compound semiconductor can be separated from a substrate used for epitaxial growth and then bonded to a conductive support substrate. This technique realizes, for example, formation of an electrode on the bottom surface of a support substrate of a light-emitting diode. Also, this technique realizes production of a light-emitting device having facing opposite electrodes (i.e., positive and negative electrodes) on both the bottom surface of a substrate and the uppermost surface of an epitaxial layer, similar to the case of a GaAs light-emitting device.
Provision of positive and negative electrodes in such a manner that they face each other via a light-emitting layer sandwiched therebetween is advantageous in that the area of a light-emitting region can be regulated to be approximately equal to the horizontal area of a support substrate, and that light extraction performance per unit device can be improved by virtue of attainment of light emission having uniform intensity. Prior art of the present invention is described in Japanese Patent Application Laid-Open (kokai) No. 2008-186959 filed by the applicant of the present invention.
In the laser lift-off technique, when, for example, an interfacial thin-film portion of a gallium nitride (GaN) layer which faces a sapphire substrate via a buffer layer formed of aluminum nitride is irradiated with a laser beam having an appropriate wavelength, GaN is decomposed into molten gallium (Ga) and nitrogen (N2) gas. When decomposition of the gallium nitride (GaN) layer is performed sequentially from a peripheral portion of a wafer, molten gallium (Ga) and nitrogen (N2) gas which are produced through the decomposition can be discharged to the periphery of the wafer.
However, for increasing irradiation energy per irradiation area, irradiating the entire wafer with laser light at a time is not preferred. Thus, for example, laser irradiation may be performed as follows. The range of a single laser irradiation is adjusted to, for example, a square measuring 0.1 mm to several mm square, and the wafer is divided into the squares. Laser irradiation is performed in a scanning manner from a peripheral portion of the wafer.
Since the volume of nitrogen gas generated in a single laser irradiation is large, the generated nitrogen gas induces large stress in such a direction as to separate an epitaxial growth substrate and an epitaxial layer bonded to a support substrate from each other.
In practice, the thickness of a GaN layer to be decomposed by ultraviolet light is considered to be on the order of several nm to several tens of nm. Thus, in a laser-irradiated area, the gap between the GaN layer and the hetero-substrate is very narrow. Meanwhile, the volume of nitrogen gas generated by laser irradiation is very large. Accordingly, if the generated nitrogen gas is not efficiently discharged outward, large stress is imposed on a region of bond between the epitaxial layer and the hetero-substrate.
In order to cope with the above problem, there is a technique in which trenches for discharging nitrogen gas are formed along, for example, device separation lines, and then laser lift-off is performed. First, this technique will be described.
FIGS. 7A to 7D are sectional views showing a conventional process of laser lift-off.
An n-type layer 11 and then a p-type layer 12 are epitaxially grown on an epitaxial growth substrate 100, thereby forming an epitaxial layer 10. A light-emitting area L is formed in an MQW structure, but is merely represented by the thick broken line in FIGS. 7A to 7D.
Next, trenches tr are formed by laser irradiation, for use as air vents at the time of laser lift-off. The trenches tr divide the epitaxial layer 10 into a large number of expanded device areas S. The expanded device areas S encompass respective device areas S′, which will be described below, and are greater in size than the device areas S′. Each of the expanded device areas S and device areas S′ has a rectangular shape (including a square shape) as viewed in plane.
Next, a conductive multilayer film 120 is formed on the surface of the p-type layer 12. The top surface of the conductive multilayer film 120 (in FIG. 7A, the bottom surface most distant from the p-type layer 12) is of a solder layer. In formation of the conductive multilayer film 120, the conductive multilayer film 120 may cover the previously formed trenches tr, so long as the trenches tr can serve as air vents for outward communication, or outward communication can be established at the time of laser irradiation for laser lift-off.
In this manner, there is formed the wafer appearing on the upper side of FIG. 7A which has the trenches tr and in which the epitaxial layer 10 and the conductive multilayer film 120 are formed on the epitaxial growth substrate 100. The epitaxial layer 10 bonded to the epitaxial growth substrate 100 is divided into a plurality of the expanded device areas S by the trenches tr.
Next, a conductive multilayer film 210 is formed on a support substrate 200 formed from a conductive material. The top surface of the conductive multilayer film 210 is of a solder layer (see the lower side of FIG. 7A).
The epitaxial growth substrate 100 having the epitaxial layer 10, and the support substrate 200 are bonded together such that the conductive multilayer films 120 and 210, whose top layers are solder layers, face each other (FIG. 7B).
Next, laser lift-off is performed. A thin-film portion of the n-type layer 11 of the epitaxial layer 10 in the vicinity of the interface between the n-type layer 11 and the epitaxial growth substrate 100 is irradiated with laser light so as to decompose the thin-film portion of the n-type layer 11. At this time, the area of a single laser irradiation (shot area, LS) encompasses a single expanded device area S. In this manner, the entire thin-film portion of the n-type layer 11 in the vicinity of the interface between the n-type layer 11 and the epitaxial growth substrate 100 of sapphire is decomposed for debonding, thereby removing the epitaxial growth substrate 100 (FIG. 7C).
Next, chip peripheries s1 and s2 of the epitaxial layer 10 are removed by dry etching. Then, n electrodes 130 are formed.
A conductive multilayer film 230 is formed on the back side of the support substrate 200 of silicon.
In this manner, the structure shown in FIG. 7D is completed. Cutting allowance areas Ct defined by the broken lines are removed, thereby yielding individual devices.
Each of the device areas S′ in FIG. 7D is smaller in size by the chip periphery s1 of the epitaxial layer 10 than the expanded device area S in FIG. 7A. Each of the cutting allowance areas Ct encompasses the chip periphery s1 and the trench tr. The chip periphery s2 is an area of the epitaxial layer 10 which is removed for allowing formation of an insulating protection film for preventing a short circuit between the n electrodes 130 and an area on a p side extending from the p-type layer 12 to the conductive multilayer film 230.
Meanwhile, the following problem associated with each of laser irradiations in FIG. 7B has been found. In each laser irradiation, stress is generated in a laser-unirradiated area, resulting in crack formation in the epitaxial layer 10 in the laser-unirradiated area. This problem is described below with reference to FIGS. 8A and 8B.
FIG. 8A is a sectional view showing a state in which an expanded device area S2 is under laser irradiation. FIG. 8B is a sectional view showing a state in which an expanded device area S3 is under laser irradiation.
As shown in FIG. 8A, four expanded device areas are denoted by S1, S2, S3, and S4. The expanded device areas S1 and S2 are separated by a trench tr12; the expanded device areas S2 and S3 are separated by a trench tr23; and the expanded device areas S3 and S4 are separated by a trench tr34. Suppose that a laser shot (LS) is performed four times in such a manner as to sequentially encompass the expanded device areas S1, S2, S3, and S4.
In FIG. 8A, the expanded device area S1 has already been laser-irradiated; the expanded device area S2 is under laser irradiation; and the expanded device areas S3 and S4 are laser-unirradiated.
At this time, nitrogen gas is generated as a result of decomposition of a portion of the n-type layer 11 in the expanded device area S2 in the vicinity of the interface between the n-type layer 11 and the epitaxial growth substrate 100. The generated nitrogen gas is discharged through the trenches tr12 and tr23. At this time, very large volume expansion of the nitrogen gas occurs. The epitaxial layer 10 in the expanded device area S1 to the left of the expanded device area S2 under laser irradiation is separated from the epitaxial growth substrate 100. Thus, the epitaxial layer 10 in the expanded device area S1 is free from imposition of large stress thereon. By contrast, the epitaxial layer 10 in the expanded device area S3 to the right of the expanded device area S2 under laser irradiation is bonded to the epitaxial growth substrate 100. Thus, large stress is imposed on the epitaxial layer 10 in the expanded device area S3. Specifically, nitrogen gas is generated as a result of decomposition of a portion of the n-type layer 11 in the expanded device area S2 in the vicinity of the interface between the n-type layer 11 and the epitaxial growth substrate 100. The generated nitrogen gas induces stress in such a direction as to lift the epitaxial growth substrate 100. As a result, a crack is highly likely to be formed, in the vicinity of the trench tr23, in the epitaxial layer 10 in the expanded device area S3 to the right of the expanded device area S2 under laser irradiation. A once-formed crack in the epitaxial layer 10 does not disappear in subsequent processes and is highly likely to progress. In the case of such progress of crack, the crack formed, in the vicinity of the trench tr23, in the epitaxial layer 10 in the expanded device area S3 progresses through the chip periphery s1 in the cutting allowance area Ct in FIG. 7D and then through the chip periphery s2 to be removed by etching, and finally reaches the device area S′, thereby greatly impairing device characteristics.
Meanwhile, referring to FIG. 8B, the epitaxial layer 10 in the expanded device area S2 to the left of the expanded device area S3 under laser irradiation is separated from the epitaxial growth substrate 100. Thus, the epitaxial layer 10 in the expanded device area S2 is free from imposition of large stress thereon.
However, in a rare case, since a large amount of nitrogen gas is generated instantaneously in the expanded device area S3, a crack may be formed in the epitaxial layer 10 in the expanded device area S2 separated from the epitaxial growth substrate 100. Particularly, in the case where the support substrate is not sufficiently fixed, stress induced by the generated gas causes vibration of the support substrate, so that a crack is likely to be formed in the epitaxial layer 10.
Japanese Patent Application Laid-Open (kokai) No. 2004-363532 discloses a technique in which laser-irradiated areas of an epitaxial growth substrate, such as a sapphire substrate, are fragmented, thereby removing the epitaxial growth substrate.
In use of the technique disclosed in Japanese Patent Application Laid-Open (kokai) No. 2004-363532, sapphire fragments are scattered. Collecting such fragments is costly.